Access to the code could give its competitors a better understanding of how Microsoft's technology works.

Microsoft said it did not yet know the source of leak, or how many people have access to it on the net, but confirmed it accounted for about 15% of the total code it uses.

The code has appeared on several underground websites and net chat rooms.

It has been widely downloaded across the net and being openly discussed on specialist technology websites.

"The source code leak spread quickly in the underground," said Ken Dunham of the security consultancy iDefense.

Microsoft said there was no indication the code leak was a result of a breach of Microsoft's corporate network.

Instead it seems the leak could be a software developer, but the company has yet to to pinpoint a suspect.

There is concern that hackers who get copies of the code could find it easier to break into systems running Windows software.

But Microsoft said that was unlikely since the code comprised of relatively small proportion of the total source code.

Too early

Security experts have said computer users and companies running Windows 2000 and NT should not be concerned about the leaked code at the moment.

The code contains valuable information about Windows

David Emm, security expert at McAfee's Avert research unit, told BBC News Online it was too early to say what the impact of the leak would be.

"It is a small amount, and Microsoft are suggesting it is not much good on its own. But it is very difficult to know whether it is something that could be used by the people to do harm," he said.

"The analogy to use is if someone wants to break into my house, they would not necessarily need a house plan. If they know there was a section of my house that had a window not covered by an alarm, that would make it easier for them."

But businesses have been advised to be vigilant and beef up their security teams.

"They should be keeping a careful eye on their IT systems and scrutinise their security systems for unusual traffic," said Tarek Meliti, technical director of server hosting company TDM Group.

It is the second security headache for the company in a week. On Tuesday, it admitted a flaw in its latest Windows operating systems that could allow hackers access to many of the hundreds of millions of computers worldwide.